Electro-optic materials (hereinafter also referred to as EO materials) are applicable to optical control elements, such as an optical modulator, an optical switch, an optical memory, an optoelectronic circuit, wavelength tuning, an electric field sensor, THz wave generation and detection, and holography. EO materials have been produced using inorganic materials, such as lithium niobate. However, inorganic materials have limited high speed performance and lack sufficient capacity to achieve the next-generation ultra-high-speed optical communication.
Organic EO polymers exhibit a larger electro-optic effect (hereinafter also referred to as an EO effect) compared with inorganic materials and achieve high-speed operation, and therefore, the polymers are expected to serve as electro-optic materials for creating the next-generation optical communication. However, application of organic EO polymers to produce devices with optical control elements has a number of problems to be solved, and such devices have not yet been used in practice. One of the problems is to achieve a long-term stability of an EO effect.
Devices with optical control elements usually have an optical waveguide. The basic structure of an optical waveguide using an organic EO polymer is usually a three-layer structure in which a core layer comprising an organic EO polymer is sandwiched by upper and lower cladding layers not comprising an organic EO polymer. In order to exhibit an EO effect, the organic EO polymer have to be subjected to alignment (poling) treatment by applying an electric field at a temperature close to the glass transition temperature (Tg) of the polymer. However, when the Tg is too low, the alignment may be relaxed in a short period of time, leading to a decrease in the EO effect.
For formation of an optical waveguide, the core layer must have a higher refractive index than those of the cladding layers. The core layer also must have an equal or higher electrical resistivity than those of the cladding layers, otherwise the electric field is not effectively applied to the core layer, resulting in poling failure.
The present inventors found a method for adjusting the refractive index and the electrical resistivity of an organic EO polymer by adjusting the concentrations of components having second-order nonlinear optical properties (i.e. electro-optic molecules) in a crosslinkable polymer composition that contains two or more components having second-order nonlinear optical properties and has a photopolymerizable residue in the polymer side chain (Patent Literature 1).
However, when the organic EO polymer produced by the method described in Patent Literature 1 is used to produce core and cladding layers, the cladding layers have a much lower Tg than the Tg of the core layer. A much higher poling temperature than the Tg of the organic EO polymer would result in softening and deformation of the EO polymer, and therefore, the poling temperature was required to be adjusted to the Tgs of the cladding layers so that the cladding layers did not deform. However, the adjusted poling temperature was too low for poling of the core layer.
The Tg of an organic EO polymer is determined by the kind of polymer that forms the backbone and the addition ratio of an EO molecule in the polymer. However, polymethyl methacrylate, which is used as a backbone polymer of a typical organic EO polymer, has a Tg as low as 100 to 110° C., which is insufficient for a device with an optical control element, the Tg of an organic EO polymer can be increased by increasing the amount of an EO molecule contained in the polymer. However, an EO molecule has to be added in an optimum amount in core and cladding layers for the formation of an optical waveguide, and therefore the Tg cannot be adjusted by adjusting the amount of an EO molecule contained in the polymer.
Accordingly, a method for adjusting the Tg of an organic EO polymer to a desired level has been required.
Non-patent Literature 1 discloses a method for increasing the Tg value of an organic EO polymer by providing a copolymer of adamantyl methacrylate with a polymethacrylate. However, in the method described in Non-patent Literature 1, a monomer having an EO molecule bound thereto is used for polymerization to give an organic EO polymer, and as a result, the reaction rates varies between a monomer having an EO molecule and a monomer not having an EO molecule. Therefore, a randomly polymerized organic EO polymer has been difficult to produce.